Sources for ionization which are used in a mass spectrometer have roughly two types: ionization sources that ionize a sample the atmospheric pressure (atmospheric pressure ionization sources), and ionization sources that ionize a sample under vacuum. The atmospheric pressure ionization source eliminates the labor required for evacuating an ionization chamber to vacuum, and is thus easy to operate. Accordingly, the atmospheric pressure ionization source has been used widely.
FIG. 1 is a schematic configuration diagram showing a mass spectrometer that includes an electrospray ionization (ESI) source that is one of the atmospheric pressure ionization sources. The mass spectrometer has the configuration of a multi-stage differential pumping system including an ionization chamber 50 maintained at atmospheric pressure and an analysis chamber 53 evacuated to a high degree of vacuum, between which first and second intermediate vacuum chambers 51 and 52 are provided having their degrees of vacuum increased in a stepwise manner. The ionization chamber 50 communicates with the first intermediate chamber 51 via a thin heated capillary 502. The first intermediate vacuum camber 51 is separated from the second intermediate chamber 52 by a skimmer 512 having a small hole at its apex. The first and second intermediate chambers 51 and 52 are evacuated by a rotary pump, so as to be maintained at a low degree of vacuum. The analysis chamber 53 is evacuated by a turbo molecular pump, so as to be maintained at a high degree of vacuum.
If the inside of the mass spectrometer is opened to an atmospheric pressure condition, it takes time to thereafter evacuate the analysis chamber 53 to a high degree of vacuum. Accordingly, when a plurality of samples are sequentially subjected to mass spectrometry, the inside of a mass spectrometer is usually maintained at vacuum even during a standby status after the completion of mass spectrometry on one sample until the initiation of mass spectrometry on a next sample.
As just described, the ionization chamber 50, the first and second intermediate vacuum chambers 51 and 52, and the analysis chamber 53 communicate with one another. This causes air to continuously flow from the ionization chamber 50 at the atmospheric pressure into the analysis chamber 53 even during the standby status. Accordingly, the vacuum pumps, i.e. the rotary pump and the turbo molecular pump, need to operate even during the standby status to maintain the first and second intermediate vacuum chambers 51 and 52 and the analysis chamber 53 at vacuum, which applies an amount of load on these pumps. Furthermore, when the heated capillary 502 that is contaminated by foreign substances is removed from the mass spectrometer for replacement, for example, the quantity of the air flowing from the ionization chamber 50 to each of the chambers subsequent to the first intermediate vacuum chamber 51 increases, which disenables each of the chambers to be maintained at vacuum. With this condition, the vacuum pumps should be stopped. In other words, the inside of the mass spectrometer is opened to the atmospheric pressure condition every time the heated capillary 502 is replaced. This causes downtime for subsequent evacuation of the analysis chamber 53 to a high degree of vacuum.
In order to solve the above problems, a shutter mechanism may be provided for disconnecting the first and second intermediate vacuum chambers 51 and 52 and the analysis chamber 53 from the ionization chamber 50. For example, a partition wall having an aperture is provided inside the first intermediate vacuum chamber 51. A shutter for opening and closing the aperture and an actuator for moving the shutter are also provided in a side close to the ionization chamber 50. With such a shutter mechanism, the aperture can be closed by the shutter and reduce the amount of load on the vacuum pump during the standby status. In addition, the heated capillary 502 can be replaced with the degree of vacuum inside the analysis chamber 53 being maintained.